P-T Conditions of Pan-African Orogeny in Southeastern Nigeria

Central European Geology, Vol. 51/4, pp. 359–378 (2008) DOI: 10.1556/CeuGeol.51.2008.4.5 P-T conditions of Pan-African orogeny in southeastern Nigeria Bassey E. Ephraim Barth N. Ekwueme Department of Geology, Niger Delta University, Department of Geology Wilberforce Island, Bayelsa State, Nigeria University of Calabar, Calabar, Nigeria Mohssen Moazzen* Monir Modjarrad Department of Geology, University of Tabriz, Department of Geology, Tabriz, Iran University of Urmia, Urmia, Iran Different rock types from the area northeast of Obudu, southeastern Nigeria were investigated in order to place constraints on their metamorphic conditions. Detailed petrographic studies indicate four main rock groups in the studied area, namely migmatitic gneiss, migmatitic schist, granite gneiss and a minor amount of amphibolite, metagabbro and dolerite. The chemistry of minerals in these rocks is used to estimate metamorphic pressure and temperature (P-T) using appropriate geothermometers and geobarometers. The estimated temperature for migmatitic gneiss of the area is ~600–625 °C and 600–650 °C for migmatitic schist; the pressure is ~ 8 kbar. For amphibolite the temperature is ~600–700 °C and pressure is 8–12 kbar. The estimated pressures and temperatures for the northeast Obudu rocks correspond to upper amphibolite to lower granulite facies metamorphism. The metamorphism occurred due to continent-continent collision during the Pan-African orogeny, most likely during the D1 deformational phase of the area. The recorded high pressures possibly resulted from crustal thickening in the area. P-T conditions for Pan-African orogeny in northeast Obudu area are in good agreement with P-T estimations for the Pan-African event in adjacent areas. Key words: Pan-African orogeny, Nigeria, Obudu, upper amphibolite-lower granulite facies Introduction The Precambrian basement terrane in Nigeria is located in the Neo-Proterozoic to Early Phanerozoic Pan-African Trans-Saharan mobile belt that stretches from North Africa to Brazil, and has boundaries with the West African Craton to the Addresses: *Corresponding author: M. Moazzen: University of Tabriz, 51664, Tabriz, Iran Phone: +98 411 339 2679, e-mail: [email protected] Received: October 20, 2008, accepted: January 22, 2009 1788-2281/$ 20.00 © 2008 Akadémiai Kiadó, Budapest 360 B. E. Ephraim et al. west and the Gabon-Congo Craton to the southeast (Torquato and Cordani 1981; Caby et al. 1981; Boullier 1991; Attoh 1998). Basement rocks occurring in Nigeria are collectively referred to as the 'basement complex', and are exposed in three main regions, namely the southwestern region (Annor 1981; De Swardt 1953; Rahaman 1973; Odeyemi 1976, 1977), the southeastern region (Rahaman et al. 1981; Ekwueme and Ekwere 1989; Ekwueme 1990) and the north-central region (Olarewaju and Rahaman 1982) (Fig. 1a, b). Four episodes of deformation that are prior to, contemporaneous with, or even post-dating metamorphism within the complex have been established on the basis of a wealth of isotopic age data acquired over the years from the basement rocks (Grant et al. 1972; Oversby 1975; Pidgeon et al. 1976; Ogezi 1977; Van Breemen et al. 1977; Grant 1978; Mullan 1979; Ajibade 1980; Fitches et al. 1985; Ekwueme 1987; Ajibade 1988; Ekwueme and Caen-Vachette 1992; Dada et al. 1993; Ferré et al. 1995; Kroener et al. 2001; Ekwueme and Kröner 2006). Accordingly, the deformation and metamorphism that have affected the basement complex are products of four thermotectonic/ orogenic events that include the Liberian (2,700 Ma), the Eburnean (2,200 Ma), the Kibaran (1,300–1,400 Ma), and the Pan-African (450–1,100 Ma) ones. Each of these events has left structural imprints on the basement rocks of the complex (Ekwueme 1994). Many workers on the Nigerian basement, including McCurry (1971) and Rahaman (1976), are of the view that the Pan-African orogenic event was the latest, most pervasive and penetrative deformation episode, and that it completely obliterated earlier structures, primary fabrics and metamorphic assemblages of the complex. On the other hand, others, including Grant (1978), Onyeagocha and Ekwueme (1982), Ekwueme (1987), Oluyide (1988) and Ekwueme (1994) favor the view that although it was pervasive, the Pan-African event did not completely homogenize the rocks of the basement, so that traces of earlier structures still remain within the complex. In southeastern Nigeria, published works on P-T evaluation of Pan-African metamorphism are scarce. Ferré et al. (2002) observed a predominantly granulite facies metamorphism for eastern Nigeria based on mineral paragenesis. However, details of the P-T estimation method is lacking in their paper. Mvondo et al. (2003) have calculated temperatures of 550 °C to 800 °C and pressures of 4 to 12 kbar for Pan-African metamorphism of schist and gneiss from Yaounde, Cameroon. The present research focused on pelitic and basic rocks, which experienced the Pan-African metamorphic event in the Obudu Plateau area of southeastern Nigeria. Petrographic features and mineral chemistry of the rocks are used to put constraints on the P-T conditions of the Pan-African orogeny in this area. Regional geologic setting Most parts of southeastern Nigeria are underlain by high-grade metamorphic rocks (Wright 1971; Ekwueme 1990, 2003; Ferré et al. 2002; Ejimofor et al. 1996). Central European Geology 51, 2008 P-T conditions of Pan-African orogeny in southeastern Nigeria 361 Sedimentary basin Younger granites Older granites Schist belt Migmatite gneiss b Metagabbro Granite gneiss Migmatitic schist Migmatitic gneiss Fig. 1 a) Geologic map of Nigeria. The studied area is shown by a rectangle. b) Simplified geologic map of the northeast Obudu area Central European Geology 51, 2008 362 B. E. Ephraim et al. The region is also often characterized by the preponderance of Pan-African granitic rocks, which account for more than 80% of the exposed bedrock, and by the general absence of well-developed schist belts similar to the ones in the southwestern and north-central regions of Nigeria. In fact, southeastern Nigeria appears to have more in common with Cameroon and the Central African fold belt than with the north-central and southwestern regions of the basement complex of Nigeria (Ekwueme 2003; Ephraim 2005). Consequently its evolutionary history is currently linked with that of the Pan-Africa mobile belt in Central Africa, north of the Congo Craton (Central African Foldbelt), also known as the Oubanguide or North Equatorial Foldbelt (Toteu et al. 2001; Toteu et al. 2004), and not with the Pan-Africa mobile belt at the margin of the West African Craton, known as the Dahomeyide Orogen. Accordingly, southeastern Nigeria has in recent times been associated with the continent–continent collision involving the northern edge of the Congo Craton as the passive margin and the Adamawa–Yadé and West Cameroon northern block as the active margin (Ephraim 2005; Toteu et al. 2004), instead of the continent–continent collision involving the passive continental margin of the West African Craton and the active continental margin of the Tuareg–Nigerian Shield/ Hoggar Craton (about 600 Ma). Field relations and petrography The area of investigation, northeast Obudu, constitutes part of the Bamenda Massif extensions into southeastern Nigeria (Fig. 1a). It is a high-grade metamorphic terrane that consists predominantly of quartzofeldspathic schist and gneiss that have been variably migmatized and intruded by rocks of acidic- intermediate-basic compositions. These rocks form parts of the chains of major intrusions that extend from the Republic of Cameroon to the margin of the Benue Trough. The chains are structurally controlled by existing North-South Pan- African trend within the basement. Rock types mapped within the northeast Obudu area, together with their structural orientations, are presented in Fig. 1b. The field/megascopic observations and the microscopic features of these rocks have been found to be consistent with its classification into four subgroups, namely: 1) Migmatitic gneiss 2) Migmatitic schist 3) Granite gneiss 4) Minor rocks (mainly amphibolite, metagabbro and dolerite) Detailed descriptions of the field and petrographic characteristics of each rock type are provided below: Central European Geology 51, 2008 P-T conditions of Pan-African orogeny in southeastern Nigeria 363 Migmatitic gneiss Quartzofeldspathic gneissic rocks that often display migmatitic characteristics constitute the dominant rock type occurring within the northeast Obudu area. The occurrence and structural orientation (the main foliation) of these rocks are shown in Fig. 1b. The migmatitic gneiss commonly occurs in intimate association with granite gneiss of the area. The heterogeneous deformation and migma- tization that may have affected the migmatitic rock group resulted in the occurrence and intimate associations of the banded, semibanded and homogeneous types in such a manner that mapping each of these varieties as distinct units become almost impossible. Megascopically, the rock generally consists of the metamorphic host rock (paleosome) and leucocratic acid injections (leucosome). The mineralogical modal composition of the migmatitic gneiss is presented in Table 1. The leucocratic mineral-rich phases are predominantly composed of variable proportion of quartz (23–33%), plagioclase (21–35%), orthoclase (7–24%) and sometimes thin selvages of biotite. On the other hand, biotite (3–33%), garnet (1–13%), sillimanite (~1%) and sericite (1–2%) occur as replacements/alteration

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